The statement that mass bends spacetime is a simplified way of describing the effects of mass on the curvature of spacetime, as described by Einstein's general theory of relativity.
In general relativity, mass and energy are not viewed as separate entities but as different manifestations of the same thing. Mass and energy are sources of gravity, and they curve the fabric of spacetime around them. This curvature is what we refer to as "bending" of spacetime.
When we say that mass bends spacetime, we mean that mass-energy tells spacetime how to curve, and the curvature of spacetime, in turn, influences the motion of particles and objects within it. Objects move along the curves determined by the curvature of spacetime.
The effect of mass on the curvature of spacetime is not merely a compression or displacement of spacetime. It is a more intricate relationship where the presence of mass distorts the geometry of spacetime in a way that objects moving through it experience what we perceive as gravitational forces.
If we were to assume a purely compressive or displacing effect of mass on spacetime without considering curvature, the resulting physics would not be consistent with observations. We wouldn't be able to explain phenomena like the bending of light around massive objects (gravitational lensing), the precession of planetary orbits, or the time dilation near massive bodies.
The concept of curved spacetime in general relativity provides a comprehensive framework that successfully explains a wide range of gravitational phenomena and is consistent with experimental observations. It is a powerful theory that has been extensively tested and confirmed through various experiments and observations.